Television viewer preference for colored motion picture over black and white motion pictures has made it desirable to color the vast stock of black and white motion pictures.
Coloring a black and white motion picture, where the motion picture is in video tape format, requires adding a color subcarrier to the black and white video signal. This coloring process is difficult due to the large number of frames that constitute a black and white motion picture. In a video tape signal derived from a black and white motion picture each frame constitutes a series of scan lines, and each scan line constitutes a series of dots or pixels. To color a black and white motion picture video signal requires assigning a value for the color subcarrier to each dot or pixel of the black and white motion picture video signal. Because of the large number of pixels in a frame and consequently in the motion picture, this process can be very time consuming.
However, the applicants have realized that motion in a black and white motion picture video tape signal corresponds to a change in the value of the luminance for a particular part of a frame from one frame to the next. The applicants have found that only a small percentage of each frame changes in this manner. The changes from one frame to the next are due to moving edges or noise. Motion is picked up only at moving edges because dots or pixels corresponding to the interior of a moving object remain at approximately the same luminance from one frame to another.
Since only a small percentage of each frame changes from one frame to another, once one frame has had colors assigned to each dot or pixel of the frame, corresponding dots or pixels in the next frame can be assigned the same color, and changes of color are only required to be made to areas corresponding to areas of motion. This significantly reduces the number of operations required to color a black and white motion picture.
It is therefore desirable to isolate parts of a video tape signal corresponding to areas of motion, without picking up noise, and to assign new colors only to those parts of the video signal corresponding to areas of motion, the other parts of the color signal remaining the same from frame to frame. By this method a color mask may be produced which when combined with the black and white video signal yields a color video signal. The original black and white film frequently contains a noise component which may detract from the pleasure of viewing the film. This noise may be reduced by averaging successive frames of the video signal since noise is random from frame to frame, (thus appearing in different places in successive frames).
Motion also appears as a frame to frame difference but it is progressive from one frame to the next. Averaging successive frames, therefore, tends to smear the motion.
It has therefore been found desirable to avoid noise reduction in parts of a frame where motion is present. This means that the areas of motion retain a noise component, but since the areas of motion are typically small as compared with stationary areas, this noise does not distract the viewer.